The present invention relates to a process for preparing phthalic anhydride by catalytic gas-phase oxidation of o-xylene/naphthalene mixtures by molecular oxygen using 2 catalyst beds comprising coated catalysts which comprise cesium, where the cesium content of the catalyst of the second zone is less than 15% by weight of the cesium content of the first zone.
The gas-phase oxidation of o-xylene and/or naphthalene to give phthalic anhydride is well known and has been widely described in the literature. It is generally carried out by passing a mixture of a gas comprising molecular oxygen, for example air, and the starting material to be oxidized through a multiplicity of tubes arranged in a reactor, with a bed of at least one catalyst being located in the tubes. To regulate the temperature, the tubes are surrounded by a heat transfer medium, for example a salt melt. Despite this thermostating, it is possible for xe2x80x9chot spotsxe2x80x9d in which the temperature is higher than in the remainder of the catalyst bed to occur. These hot spots give rise to secondary reactions such as the total combustion of the starting material or they lead to the formation of undesirable by-products which can be separated from the reaction product only with great difficulty, if at all, for example the formation of phthalide or naphthoquinone. To weaken these hot spots, it has already been proposed that catalysts of different activity be arranged in zones in the catalyst bed, with the less active catalyst generally being arranged toward the gas inlet end and the more active catalyst being arranged toward the gas outlet from the catalyst bed. The catalysts of different activity in the catalyst bed can be exposed to the reaction gas at the same temperature, but the two zones of catalysts of different activity can also be thermostated to different reaction temperatures for contact with the reaction gas.
Catalysts which have been proven useful for the preparation of phthalic anhydride are coated catalysts (see below).
The oxidation of naphthalene or o-xylene is, owing to the different reactivities of the starting materials, generally carried out using different catalysts, so that when o-xylene/naphthalene mixtures are employed, it is necessary to use specifically adapted-catalysts which are suitable both for the oxidation of o-xylene and the oxidation of naphthalene, as is described, for example, in BE 893521, EP 286448 and DE 2238067.
A specific solution to this problem is proposed in EP 539878, which states that, when using catalysts of the inlet stage in a bed height of 15-85%, based on the volume of the total catalyst bed, and a catalyst of the subsequent stage in a bed height of 85-15%, based on the volume of the total catalyst bed, in the direction of the gas outlet, in the form of superposed zones, very good results are achieved when, in the catalyst of the inlet stage, a catalyst substance is applied to an inactive support in a ratio in the range from 5 to 20 g/100 ml, where the catalytically active substance comprises from 1 to 20% by weight of V2O5 and from 99 to 80% by weight of titanium dioxide of the anatase type having a specific surface area of from 10 to 60 m2/g, with incorporation of, based on 100 parts by weight of the total amount of the abovementioned two components, from 0.01 to 1% by weight of Nb2O5, from 0.2 to 1.2% by weight of P2O5, from 0.5 to 5% by weight of Sb2O3 and from 0.3 to 1.2% by weight of at least one compound selected from the group consisting of oxides of potassium, cesium, rubidium and thallium, where in the catalyst of the second zone the amount of a compound selected from the group consisting of compounds of potassium, cesium, rubidium and thallium is from 17 to 63% by weight of the amount of the corresponding compound in the catalyst of the inlet stage. Outside this xe2x80x9cwindowxe2x80x9d (variation of the length ratio of the second catalyst to the total length within the range 85-15% and the alkali metal ratio within the range 17-63%), the reaction is, according to this patent, xe2x80x9cdifficultxe2x80x9d or associated with relatively high yield losses.
It is an object of the present invention to remedy these deficiencies.
We have found that this object is achieved by using catalysts of virtually the above composition in which the alkali metal present is cesium and the cesium content of the second zone is less than 15% by weight of the cesium content of the first zone. Such catalysts have particularly advantageous properties compared to the above-described catalysts in the specified xe2x80x9cwindowxe2x80x9d.
Specifically, we have found an improved process for preparing phthalic anhydride by catalytic gas-phase oxidation of o-xylene/naphthalene mixtures by molecular oxygen using
a catalyst I in a first zone on the gas inlet side which makes up from 25 to 75 percent by volume of the total catalyst volume, comprising, in each case based on the catalytically active composition, from 1 to 10% by weight of vanadium oxide (calculated as V2O5), from 1 to 10% by weight of antimony oxide (calculated as Sb2O3) and from 80 to 98% by weight of titanium dioxide of the anatase type having a BET surface area of from 13 to 28 m2/g and also from 0.05 to 1% by weight of cesium (calculated as Cs) applied to a steatite support and
a catalyst II in a second zone which makes up the remaining 75 to 25 percent by volume of the total catalyst volume, comprising, in each case based on the catalytically active composition, from 1 to 10% by weight of vanadium oxide (calculated as V2O5), from 1 to 10% by weight of antimony oxide (calculated as Sb2O3) and from 80 to 98% by weight of titanium dioxide of the anatase type having a BET surface area (cf. J. Amer. Chem. Soc. 60 (1938), 309 et seq.) of from 13 to 28 m2/g and from 0.01 to 1% by weight of phosphorus oxide (calculated as P) and also from 0.01 to 0.2% by weight of cesium (calculated as Cs) applied to a steatite support, wherein the cesium content of the catalyst II is less than 15% by weight of the cesium content of the catalyst I and the catalyst I and the catalyst II have been prepared without addition of compounds of niobium.
The reduction of the cesium content of the catalyst II to less than 15% by weight of the cesium content of the catalyst I enables the product quality to be significantly increased by reducing the content of the undesirable by-products phthalide and naphthoquinone without significantly influencing the phthalic anhydride yield. This is especially pronounced at the relatively low bath temperatures which are necessary to achieve space velocities over the catalyst of more than 60 g/standard m3. This effect is particularly noticeable when the cesium content of the catalyst II is 10-13% by weight of the cesium content of catalyst I and the proportion of the total catalyst bed occupied by the zone of the catalyst II is 40-60%.
The catalysts used can, even without doping, further comprise small amounts of metal oxides such as those of niobium, tungsten and/or lead. These arise via possible impurities in the commercial anatase used and should thus not be regarded as doping of active compositions. Further impurities, in particular alkali metal impurities, are generally less than 0.01% by weight.
Catalysts which can be used in the process of the present invention are, within the limits defined in the claims, coated catalysts known per se, as are described in the specialist literature for preparing phthalic anhydride. The composition and production of such xe2x80x9cstandard catalystsxe2x80x9d can be summarized as follows:
in general, catalysts which have found to be useful are coated catalysts in which the catalytically active composition (xe2x80x9cactive compositionxe2x80x9d) is applied in the form of a shell to a support material which is generally inert under the reaction conditions, e.g. quartz (SiO2), porcelain, magnesium oxide, tin dioxide, silicon carbide, rutile, alumina (Al2O3), aluminum silicate, magnesium silicate (steatite), zirconium silicate or cerium silicate or mixtures thereof. The catalytically active constituents of the catalytically active composition of these coated catalysts are generally titanium dioxide in its anatase modification plus vanadium pentoxide. In addition, the catalytically active composition can further comprise small amounts of many other oxidic compounds which act as promoters to influence the activity and selectivity of the catalyst by reducing or increasing its activity. Examples of such promoters are the alkali metal oxides, in particular lithium, potassium, rubidium and cesium oxide, thallium(I) oxide, aluminum oxide, zirconium oxide, iron oxide, nickel oxide, cobalt oxide, manganese oxide, tin oxide, silver oxide, copper oxide, chromium oxide, molybdenum oxide, tungsten oxide, iridium oxide, tantalum oxide, niobium oxide, arsenic oxide, antimony oxide, cerium oxide and phosphorus pentoxide. Promoters which reduce the activity and increase the selectivity are, for example, the alkali metal oxides, whereas oxidic phosphorus compounds, in particular phosphorus pentoxide, increase the activity of the catalyst but reduce its selectivity.
Such coated catalysts are produced, according to the processes of DE-A 1642938 and DE-A 1769998, by spraying an aqueous and/or organic solvent-containing solution or suspension of the constituents of the active composition and/or their precursor compounds, hereinafter referred to as the xe2x80x9cslurryxe2x80x9d, onto the support material in a heated coating drum at elevated temperature until the amount of active composition as a proportion by weight of the total catalyst has reached the desired value. According to DE 2106796, the coating procedure can also be carried out in fluidized-bed coaters as are described, for example, in DE 1280756. However, both spraying in the coating drum and coating in a fluidized bed result in high losses since considerable amounts of the slurry are converted into a mist or parts of the previously applied active composition are rubbed off again by abrasion and are carried out by the waste gas. The proportion of active composition in the total catalyst should generally only deviate slightly from the specified value, because the amount of active composition applied and thus the thickness of the coating considerably influence activity and selectivity of the catalyst. Thus, the catalyst has, in the production methods indicated, to be cooled, taken from the coating drum or the fluidized bed and reweighed at frequent intervals to determine the amount of active composition applied. If too much active composition is deposited on the catalyst support, subsequent, careful removal of the excess active composition is generally not possible without adversely affecting the strength of the shell, in particular without crack formation in the catalyst shell.
One way of reducing this problem, which has also been employed in industry, is to add organic binders, preferably copolymers of vinyl acetate/vinyl laurate, vinyl acetate/acrylate, styrene/acrylate, vinyl acetate/maleate or vinyl acetate/ethylene, advantageously in the form of an aqueous dispersion, to the slurry, using binder amounts of 10-20% by weight, based on the solids content of the slurry (EP-A 744214). If the slurry is applied to the support without organic binders, coating temperatures above 150xc2x0 C. are advantageous. when the above-described binders are added, the usable coating temperatures are, depending on the binder used, from 50 to 450xc2x0 C. (DE 2106796). The binders applied burn off within a short time after introducing the catalyst into the reactor and starting up the reactor. The addition of binder has the additional advantage that the active composition adheres well to the support so that transport and charging of the catalyst are made easier.
Further suitable methods of producing coated catalysts for the catalytic gas-phase oxidation of aromatic hydrocarbons to give carboxylic acids and/or carboxylic anhydrides are described in WO-98/00778 and EP-A 714700. The layer containing catalytically active metal oxides is applied to a support material by first preparing a powder from a solution and/or a suspension of the catalytically active metal oxides and/or their precursor compounds, if desired in the presence of auxiliaries for catalyst production, subsequently applying this to the support, if desired after prior conditioning or, if desired, after heat treatment to generate the catalytically active metal oxides, and subjecting the support which has been coated in this way to a heat treatment to generate the catalytically active metal oxides or a treatment to remove volatile constituents.
To carry out the gas-phase oxidation, the reaction tubes of the reactor, which are thermostated from the outside to the reaction temperature, for example by means of salt melts, are first charged with the catalysts. The reaction gas is passed over the prepared catalyst bed.
The reaction gas fed to the catalyst is generally produced by mixing a gas comprising molecular oxygen, which can further comprise suitable reaction moderators and/or diluents such as steam, carbon dioxide and/or nitrogen in addition to oxygen, with the aromatic hydrocarbon to be oxidized. The gas comprising the molecular oxygen generally comprises from 1 to 100 mol%, preferably from 2 to 50 mol % and particularly preferably from 10 to 30 mol %, of oxygen, from 0 to 30 mol %, preferably from 0 to 10 mol %, of steam and from 0 to 50 mol %, preferably from 0 to 1 mol %, of carbon dioxide and nitrogen as the balance. To produce the reaction gas, the aromatic hydrocarbon to be oxidized is generally introduced into the gas comprising the molecular oxygen in an amount of from 30 to 150 g per standard m3 of gas.
The o-xylene/naphthalene mixture to be oxidized can be used in a broad mixing ratio, for example in a weight ratio of from 1:99 to 99:1, but especially in such a ratio that the o-xylene content is at least 30% by weight, preferably at least 50% by weight and in particular at least 70% by weight. The ratio of o-xylene/naphthalene can remain constant over the catalyst life or can be altered. Thus, for example, the content of o-xylene can advantageously be increased over the course of time, thereby achieving a higher yield.
The reaction is generally carried out at from 300 to 450xc2x0 C., preferably from 320 to 420xc2x0 C. and particularly preferably from 340 to 400xc2x0 C., and at a gauge pressure of from 0.1 to 2.5 bar, preferably from 0.3 to 1.5 bar, using a space velocity of generally from 750 to 500 hxe2x88x921.
The preparation of phthalic anhydride over the coated catalysts to be used according to the present invention is otherwise carried out in a manner known per se, as described specifically, for example, by K. Towae, W. Enke, R. Jxc3xa4ckh and N. Bhargawa in xe2x80x9cPhthalic Acid and Derivativesxe2x80x9d, Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A20, 1992, 181.